Ch.11 - Acid-Base Titrations Flashcards
What is a solution?
A solution is a uniform mixture of 2 substances.
What are the various units of concentration?
The units of concentration include: moles per litre (M) (molarity), grams of solute per litre (gl-1), parts per million (p.p.m.), percentage as weight/weight, weight/volume and volume/volume.
How do you calculate the number of moles in a molecule?
Number of moles = volume in cm3 × concentration in M/1000.
What is a standard solution?
A standard solution is a solution whose concentration is accurately known.
What are the properties of a primary standard?
A primary standard is a water-soluble substance that is stable and available in pure form.
What is needed in titration calculations?
In titration calculations, where only the unknown concentration is required, we need to know: the balanced equation (for the mole ratio of the 2 reagants), the concentration of one of the reagants and the volume of both reagants.
How must you minimise error when making up a standard solution? (Preparation of a standard solution of sodium carbonate)
A volumetric flask is used, because when its filled to the calibration mark (at specified temperature), it contains a specific known volume.
Electronic mass balances are used to measure the mass of solutes because they are accurate.
When the solute is being dissolved in a beaker, or transferred from the beaker to a volumetric flask, none can be lost. A wash bottle is useful for this purpose.
How is a solution of known concentration made up?
The solute is weighed accurately. It is transferred into a beaker and pure water is added. The mixture is stirred until the solute has dissolved. Solution is washed off the stirring rod, using a wash bottle. Solution is poured into a clean, volumetric flask. Using pure water, the beaker is washed out, and the washings are added to the volumetric flask. Nearing the calibration mark, water is added dropwise, till the bottom of the meniscus lies on the mark. The flask is sealed and inverted several times to ensure homogeneity.
What does the shade of colour represent?
Diluting the solution results in the lightening of the colour of the solution. The deeper the colour, the more concentrated the solution.
What is a pipette?
A pipette is used to place a specific, known value of solution in the conical flask.
How can a pipette be used accurately?
When filled to the mark, a pipette contains slightly more than this volume, however some of this is retained. To ensure that the pipette delivers the liquid accurately, release its contents freely, then touch its tip to the inside of the conical flask to deliver any residual liquid.
What are some precautions to ensure accurate titration results?
The burette and pipette is washed out with pure water and then with the solution that it is to contain.
The tap of the burette is briefly opened to fill the part of the burette below the tap so that the solution displaces air.
The burette is filled past the 0cm^3 mark.
The tap is opened again to allow the level of liquid to fall to a definite mark. The tap is then closed.
Burette readings are to be taken from eye-level with the bottom of the meniscus at the mark.
The conical flask is washed out with pure water only.
Only a few drops of indicator are added. This is because indicators are weak acids or bases; adding too much indicator would effect accuracy.
A white tile is placed under the conical flask.
The conical flask is swirled well after each addition of solution from the burette.
A rough titration is carried out for approximation.
Then 2 accurate titrations are taken, these results should agree +/- 0.1cm^3. The mean of the accurate titres are the final result.
Burettes and pipettes are used rather than graduated cylinders and beakers because they are more accurate.
What indicator is used in the standardisation of a hydrochloric acid solution using a standard solution of sodium carbonate?
Methyl orange indicator is used. At the end-point, when neutralisation just occurs, the indicator changes colour from yellow to pink. The same occurs when used in a hydrochloric acid/sodium hydroxide titration.
What indicator is used in the determination of the concentration of ethanoic acid in vinegar?
Phenolphthalein indicator is used. At the end point, the indicator changes from pink to colourless.
What else can acid-base titrations be used to find?
Acid-base titrations can be used to find the molar mass of an acid/base and the amount of water of crystallisation in a substance such as hydrated sodium carbonate.
